For the Spanish humanist Juan Páez de Castro (1510–1570) an empire was only as good as its library. In his ambitious Memorial sobre las librerias (ca. 1556) he hoped to convince the recently crowned Phillip II to found a public library to serve the many needs of the Spanish monarchy. This article analyzes the goals of his scheme and its relationship to imperial governance. Knowledge coming from Spain’s New World domains had a fundamental place in Páez de Castro’s scheme. The library would gather materials from all the crowns under the jurisdiction of the Spanish king, organize them into categories and integrate the Old and New Worlds into a single comprehensive narrative.